The present invention relates generally to wireless networks and, particularly, the present invention relates to a local oscillator (LO) architecture in a frequency translating repeater. In particular, practical considerations must be addressed when implementing such a repeater in a system where many or all the components of the repeater including the LO circuits are implemented in an integrated circuit. One important practical consideration is the degree of on-chip isolation between the receive channel and the transmit channel in the integrated circuit.
In most frequency translating repeater systems, isolation between the transmit signal path and the receive signal path is a major concern. In particular, signal input stages and even auxiliary circuit input stages, such as LO stages, are susceptible to any in-band signal energy thus signal energy from other than the intended input signal can cause signal anomalies such as interference or “jamming” as will be appreciated. For example, if the LO used for frequency down-conversion is allowed to leak into the LO input of the frequency up-converter, a signal image will be transmitted that will have a jamming effect in the receiver. Further, if the LO used for frequency up-conversion in the transmit path is allowed to leak into the receiver frequency down-converter, the transmitted signal will be down-converted into the receive band and will also have a jamming effect.
The most common way in which signal energy from LO circuits can be cross coupled is via high-Q tank circuits, LC circuits tuned to resonate at particular frequencies. Since 80 dB of isolation is typically required between the transmit LO and the receive LO, and since, from a practical standpoint, 80 dB of isolation is generally considered nearly impossible between two such tank circuits on a single semiconductor chip, then the LO circuits for the transmit path and the receive path must be at different frequencies. A second path for leakage of signal energy sufficient to couple into LO stages is through the chip substrate. While the signal energy coupled through the substrate is typically at a much lower level than coupling through tank circuits as described, achieving 80 dB of isolation between two different LO circuits on the same substrate is still difficult.
Broad-band phase noise is another form of signal anomaly leading to receiver desensitization. A typical system, with a noise figure of, for example, 8 dB, can have a resulting system noise floor at −166 dBc/Hz. Thus, if an LO in the system has a broad-band phase noise of −150 dBc/Hz, which is above the noise floor, the phase noise will be imparted to the up-converted signal when passing through the up-converter. Further, if the output of a mixer associated with the LO is −10 dBm, the total phase noise that will be input to the power amplifier (PA) will be at around −160 dBm/Hz. The PA typically has a gain of 25 dB with a noise figure of 6 dB, moving the noise level up to around −135 dBm/Hz. Given a receiver to transmitter isolation of 30 dB, the resulting leakage noise at the system input would be −165 dBm/Hz. It should be appreciated that since the LO signal and the up-converted signal are typically not coherent, they do not directly add. The resulting leakage noise of −165 dBm/Hz as described above results in about 1–1.5 dB of desensitization. Accordingly, with LO frequency offsets at greater than, for example 10 MHz, LO broad-band noise levels above −150 dBc/Hz result in 1 for 1 receiver desensitization. In other words with noise levels at, for example, −149 dBc/Hz, the system is desensitized by 2 dB, with a noise level at −148 dBc/Hz the system is desensitized by 3 dB, and so on.
Still another problem associated with any switched LO architecture is pulling. Pulling is related to instability of the LO due to changes in the output impedance presented to them. LO pulling will cause disruption in the signal being mixed with the LO until the LO settles back on frequency. It will be appreciated that the amount of time associated with LO pulling is a function of the amount of impedance change and the loop bandwidth. In certain instances, for example in 802.11 signal scenarios, because the 802.11(g) signal has a very short preamble typically 8 μs long, even small amounts of pulling can be catastrophic. Thus an exemplary LO circuit would need to have LO settling within 1 us in order to prevent loss of signal lock or the like.
An example of isolation in a repeater using frequency translation can be found in U.S. patent application Ser. No. 10/529,037 listed and incorporated above. It should be noted however, that in order to ensure robust and effective operation, a frequency translating repeater must be capable of rapidly detecting the presence of a signal and must operate cooperatively in the environment in which it is repeating by providing adequate isolation from signal energy including energy from oscillators and the like, in order to effectively repeat the transmission.